The aim of the proposed research is to examine in detail the genetic consequences of the repair of damaged DNA in irradiated and unirradiated cells of the eucaryote, Saccharomyces cerevisiae. The chief method by which this will be achieved entails the combined use of radiation sensitive strains, that are presumed to be deficient in some aspect of repair, and the iso-l-cytochrome c gene-protein system developed by Drs. Sherman and Stewart. This system, the only one available in eucaryotes, provides a powerful means of precisely delineating the DNA alterations responsible for a given mutation, either by analysis of revertant proteins or by the use of mutations defined by this method. This experimental approach will be used to examine the influence of unique nucleotide sequences on radiation mutagenesis, the specific action of the different repair enzymes, and the influence of repair deficiencies on the type and rate of spontaneous mutation, both during mitosis and meiosis. Finally, temperature conditional radiation sensitive mutants will be used to examine the relationship between repair, DNA replication and recombination.